Polishing method and apparatus, and device fabrication method

ABSTRACT

The present invention concerns a polishing method of keeping a polishing pad under a predetermined pressure and in contact with a polished surface of a wafer and performing polishing while rotating the wafer and the polishing pad, in which symmetry of a film on an alignment mark for alignment of the wafer is measured after an end of the polishing of the wafer or in the middle of the polishing and in which polishing thereafter is carried out by controlling rotation speeds of the wafer and polishing pad according to the symmetry of the film on the alignment mark, thus measured, whereby the film on the alignment mark of the wafer can be isotropically polished in symmetry.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a method and apparatus for polishing of substrates such as semiconductor wafers of Si, GaAs, InP, and so on, silica and glass substrates on the surface of which a plurality of islandlike semiconductor regions are formed, or the like, and to a device fabrication method.

[0003] 2. Related Background Art

[0004] With progress in microminiaturization and multilevel metalization of semiconductor devices, there are needs for highly precise flattening of outer surfaces of the substrates such as the semiconductor wafers of Si, GaAs, InP, and so on, the silica and glass substrates on the surface of which a plurality of islandlike semiconductor regions are formed, or the like. Further, global flattening of the outer surfaces of substrates is also demanded from the emergence of SOI wafers and the necessity for three-dimensional integration.

[0005] Chemical mechanical polishing (CMP) systems, for example, as shown in FIG. 11 and FIG. 12, are conventionally known as the flattening technology capable of microflattening as well as the global flattening of such substrates.

[0006] Describing the chemical mechanical polishing system shown in FIG. 11, the system has a wafer chuck 101 for holding a wafer W as a work with a polished surface thereof facing down, and a polishing table 105 which is placed so as to face the wafer W held on the wafer chuck 101 and to which a polishing pad P, e.g., of polyurethane, having the diameter greater than that of the wafer W, is bonded, and is further provided with an abrasive supply 108 for supplying abrasive (slurry) 107 onto the polishing pad P. The polishing pad P is mainly made of a material having unevenness in the surface, or a porous material, and grid-like grooves for supply and discharge of the abrasive to and from the wafer W are scribed in the surface of the polishing pad P.

[0007] In the chemical mechanical polishing system constructed in this structure, the polished surface of the wafer W held on the wafer chuck 101 is brought into contact onto the polishing pad P bonded to the polishing table 105, the wafer W and polishing pad P are rotated respectively in directions of arrows by unrepresented driving means while the wafer W is kept under a predetermined pressure, and, at the same time as it, the abrasive 107 is dropped from the abrasive supply 108 onto the polishing pad P, thereby polishing the polished surface of the wafer W. Concerning the driving of the wafer W and the polishing pad P, when rotational speeds (rpms) of the wafer W and the polishing pad P are set equal to each other, the linear velocity of the polishing pad P becomes constant at an arbitrary position on the wafer W, which is desirable to the global flattening. However, the grid-like groove pattern in the surface of the polishing pad P is transferred onto the polished surface of the wafer W, which ends in failure of microflattening. For this reason, it is common practice to perform polishing with a shift of several % between the rotational speeds of the wafer W and the polishing pad P.

[0008] On the occasion of this polishing, the abrasive (slurry) is used for the purpose of increasing polishing amounts and is, for example, an aqueous alkaline solution or the like in which fine particles of SiO₂ of micron order to submicron order are dispersed in a stable state.

[0009] The chemical mechanical polishing system shown in FIG. 12 has a wafer chuck 201 for holding a wafer W as a work with a polished surface thereof facing up, a wafer table 202 for supporting the wafer chuck 201, and a polishing pad holder 205 which is disposed above the wafer table 202 so as to face the wafer W held on the wafer table 202 and which holds a polishing pad P having the diameter smaller than that of the wafer W, and is arranged to supply the abrasive from the unrepresented abrasive supply in communication with a small hole provided in the polishing pad P, through the small hole to between the wafer W and the polishing pad P. In the chemical mechanical polishing system constructed in this structure, the polishing pad P of the smaller diameter held on the polishing pad holder 205 is brought into contact with the polished surface of the wafer W held through the wafer chuck 201 on the wafer table 202, the polishing pad P is rotated in a direction of an arrow and rocked by unrepresented driving means while the polishing pad P is kept under a predetermined pressure, and, at the same time as it, the abrasive is supplied from the unrepresented abrasive supply to between the polishing pad P and the wafer W, thereby polishing the polished surface of the wafer W.

[0010] The semiconductor substrate such as the wafer or the like having been polished by the polishing apparatus as described above is conveyed through a cleaning step to a next device production step and is then subjected to sequential processing steps to produce devices. FIG. 13 shows a typical flow of device production steps.

[0011] In FIG. 13, step S101 (oxidation) is a step of oxidizing the surface of the wafer, step S102 (CVD) a step of forming an insulating film on the wafer surface, step S103 (formation of electrodes) a step of forming electrodes on the wafer by sputtering or CVD, and step S104 (ion implantation) a step of injecting ions into the wafer. Then step S105 (CMP) is a step of chemically and mechanically polishing the surface of the wafer, step S106 (resist process) a step of applying a resist onto the wafer, step S107 (exposure) a step of transferring a circuit pattern of a mask into an array of shot areas in the wafer by a semiconductor exposure system of a stepper or the like, step S108 (development) a step of developing the exposed wafer, step S109 (etching) a step of removing portions except for the resist image developed, and step S110 (resist peeling) a step of removing the resist becoming unnecessary after completion of the etching. The semiconductor devices are produced by repeating these steps approximately 10 to 20 times as needed. Namely, the semiconductor substrate such as the wafer or the like after completion of the CMP step (step S105) is fed into the exposure step using an overlay inspection system, the stepper, and so on.

[0012] Incidentally, in addition to patterns p forming semiconductor devices, alignment marks m for positioning of an alignment detecting system of the overlay inspection system, the stepper, or the like are arranged on the polished surface of the substrate such as the wafer or the like, as illustrated in FIG. 14. In the polishing step, since a film s consisting of at least one layer of insulator (dielectric) or the like is also formed, as shown in FIG. 14, on the alignment marks m, the film on the alignment marks is simultaneously polished together with the film on the patterns p forming the semiconductor devices. In general, the lengths of the device patterns p to be flattened by CMP are fine patterns of not more than 1 μm, but the lengths of the alignment marks m are approximately 30 to 100 μm. Since in the CMP step the surface is polished with the pad of a viscoelastic material like a polyurethane pad, non-dense portions of unevenness are not flatly polished because of deformation of the pad during polishing, thereby causing phenomena of so-called dishing and thinning. For this reason, there remains slight unevenness in the film layer on the alignment patterns m even after the flattening by CMP, and reflection from this slightly remaining unevenness affects alignment.

[0013] Describing this in further detail, most of methods now practically used in the alignment detection systems of the overlay inspection systems, steppers (exposure systems), and the like are bright field image processing methods, and the alignment detection systems of this type are constructed as schematically shown in FIG. 15 and arranged to detect an alignment mark m formed on the wafer W, form an image thereof on CCD 301 as an image pickup device by an optical system 303, and perform various signal processes on an electric signal of the image, thereby implementing position detection. In FIG. 15, numeral 302 designates a light source of the alignment detection system, 303 an optical system including a beam splitter of the alignment detection system, and 305 and 306 a reticle and a projection lens of the stepper.

[0014] The imaging performance most required of the optical systems of the alignment detection systems of this type is symmetry of image. However, if the film s on the alignment mark m is asymmetric as shown in FIG. 16A, light (A, B, C, D,. . . ) normally incident to the alignment mark m is reflected at different angles as shown in FIG. 16A and the reflected light (A′, B′, C′, D′,. . . ) travels through the alignment optical system to be focused on the CCD as an image pickup device, as shown in FIG. 16B. Since the angles of reflection are different, positional deviation can occur because of distortion of the image on the CCD. This becomes a factor of degrading the alignment accuracy.

[0015] With the conventional polishing systems, as described above, there was the problem that the asymmetric polishing of the film on the alignment marks degraded the positioning accuracy in the alignment detection systems of the overlay inspection systems, the steppers, and so on. A conceivable reason of this asymmetric polishing is that the film on the alignment marks is not polished isotropically to become asymmetric because of execution of polishing with the shift of several % between the rotational speeds of the substrate, such as the wafer or the like, and the polishing pad, occurrence of deformation of the polishing pad with a lapse of time, and so on.

[0016] When the wafer surface is polished with the arbitrary shift of several % between the rotational speeds of the wafer and polishing pad, there occurs positional deviation of the alignment marks on the wafer because of the difference between numbers of rotations of the wafer and the polishing pad and directions of the positional deviation are as shown in FIG. 17. Namely, if there is the difference between numbers of rotations of the wafer and polishing pad, the wafer will not be polished isotropically.

SUMMARY OF THE INVENTION

[0017] An object of the present invention is to provide a polishing method and polishing apparatus of semiconductor substrate that can polish the film layer on the alignment marks for positioning of the overlay inspection system, the stepper, and the like in symmetric shape to improve positioning accuracy thereby and that can prevent the transfer of the grid-like groove pattern scribed in the surface of the polishing pad, thereby improving microflatness.

[0018] A polishing method of a substrate according to the present invention is a polishing method of keeping a polishing pad under a predetermined pressure and in a state of contact with a device forming surface of a substrate and performing polishing while rotating each of the substrate and the polishing pad, wherein symmetry of a film layer on an alignment mark for alignment of the substrate is measured after an end of the polishing of the substrate or in the middle of the polishing and wherein a rotation speed of the substrate and/or the polishing pad is controlled according to the symmetry of the film layer on the alignment mark, thus measured.

[0019] In the polishing method of the substrate according to the present invention, the control is preferably carried out so as to change the rotation speeds of the substrate and the polishing pad and equate total numbers of rotations of the substrate and the polishing pad from a start of the polishing to the end of the polishing.

[0020] In the polishing method of the substrate according to the present invention, it is preferable to set the rotation speed of the substrate, the rotation speed of the polishing pad, a polishing period, and the polishing pressure, further set a rotation speed switching time, perform calculation so as to equate the total numbers of rotations of the substrate and the polishing pad from the start of the polishing to the end of the polishing, based on the rotation speed of the substrate, the rotation speed of the polishing pad, the polishing period, and the rotation speed switching time thus set, and drive the substrate and the polishing pad at respective rotation speeds based on the result of the calculation.

[0021] In the polishing method of the substrate according to the present invention, it is preferable to store the rotation speed switching time and either one or both of the rotation speeds of the substrate and the polishing pad and the control is preferably carried out so as to equate the total numbers of rotations of the substrate and the polishing pad from the start of the polishing to the end of the polishing.

[0022] A polishing apparatus of a substrate according to the present invention is a polishing apparatus comprising driving means for rotating a substrate and driving means for rotating a polishing pad and constructed to keep the polishing pad under a predetermined pressure and in a state of contact with a device forming surface of the substrate and perform polishing while rotating each of the substrate and the polishing pad, the polishing apparatus comprising measuring means for measuring symmetry of a film layer on an alignment mark for alignment of the substrate or an input portion for inputting symmetry of a film layer on an alignment mark, and control means for controlling a rotation speed of the substrate and/or the polishing pad according to the symmetry of the film layer on the alignment mark, thus measured or inputted.

[0023] In the polishing apparatus of the substrate according to the present invention, the control means preferably performs the control so as to change the rotation speeds of the substrate and the polishing pad and equate total numbers of rotations of the substrate and the polishing pad from a start of the polishing to an end of the polishing.

[0024] In the polishing apparatus of the substrate according to the present invention, preferably, the control means comprises first input means for setting the rotation speed of the substrate, the rotation speed of the polishing pad, a polishing period, and the polishing pressure and second input means for setting a rotation speed switching time, and the control means further comprises an operation part for performing calculation so as to equate the total numbers of rotations of the substrate and the polishing pad from values inputted by the first and second input means, and a control part for driving the substrate and the polishing pad at respective rotation speeds based on the rotation speeds inputted by the first input means and the result of the calculation by the operation part.

[0025] In the polishing apparatus of the substrate according to the present invention, it is preferable to comprise a memory part for storing the rotation speed switching time and either one or both of the rotation speeds of the substrate and the polishing pad and to perform the control so as to equate the total numbers of rotations of the substrate and the polishing pad from the start of the polishing to the end of the polishing.

[0026] According to the present invention, the symmetry of the film layer on the alignment mark for alignment of the substrate is measured after the end of the polishing of the substrate or in the middle of the polishing and the rotation speed of the substrate and/or the polishing pad is controlled according to the symmetry of the film layer on the alignment mark, thus measured, whereby it becomes feasible to overcome the problem due to the difference between the rotation speeds of the substrate and the polishing pad and the temporal change of the polishing pad and to symmetrically and isotropically polish the film layer on the alignment mark for alignment on the polished surface of the substrate such as the wafer or the like.

[0027] Further, the rotation speeds of the substrate and the polishing pad are changed and the rotation speeds of the substrate and polishing pad are controlled so as to equate the total numbers of rotations of the substrate and the polishing pad from the start of the polishing to the end of the polishing, whereby the polishing can be carried out further more isotropically.

[0028] This enables symmetric polishing of the film layer on the alignment mark and can improve the alignment accuracy and further prevent the transfer of the grid-like groove pattern scribed in the surface of the polishing pad, thereby improving the microflatness.

BRIEF DESCRIPTION OF THE DRAWINGS

[0029]FIG. 1 is a layout diagram to show the schematic structure of a polishing apparatus of substrate according to the first embodiment of the present invention;

[0030]FIG. 2 is a view showing the structure of a polishing mechanical section in the polishing apparatus of substrate according to the first embodiment of the present invention;

[0031]FIG. 3 is a view showing the structure of the main part of the polishing mechanical section in the polishing apparatus of substrate according to the first embodiment of the present invention;

[0032]FIG. 4 is a flowchart of a polishing method in the first embodiment of the present invention;

[0033]FIG. 5 is a schematic diagram of an alignment mark symmetry detecting system for measuring symmetry of a film layer on an alignment mark in the polishing apparatus of substrate according to the first embodiment of the present invention;

[0034]FIGS. 6A and 6B are drawings explaining a state of measuring symmetry of a film layer on an alignment mark by the alignment mark symmetry detecting system;

[0035]FIG. 7 is a timing chart for explaining a polishing method in the second embodiment of the present invention;

[0036]FIG. 8 is a flowchart of the polishing method in the second embodiment of the present invention;

[0037]FIG. 9 is a timing chart for explaining a polishing method in the third embodiment of the present invention;

[0038]FIG. 10 is a flowchart of the polishing method in the third embodiment of the present invention;

[0039]FIG. 11 is a schematic view of a conventional chemical mechanical polishing apparatus;

[0040]FIG. 12 is a schematic view of another conventional chemical mechanical polishing apparatus;

[0041]FIG. 13 is a flowchart showing ordinary device production steps;

[0042]FIG. 14 is a schematic, cross-sectional view of a semiconductor device in the step of forming the semiconductor device;

[0043]FIG. 15 is a schematic view showing an ordinary alignment detecting system;

[0044]FIGS. 16A and 16B are drawings showing the relation between an alignment mark and an image on an image pickup device such as CCD, based on asymmetry of a film layer on the alignment mark of wafer; and

[0045]FIG. 17 is a drawing showing directions of positional deviation of alignment marks on a wafer, appearing when the surface of the wafer is polished with a shift of several % between rotation speeds of the wafer and the polishing pad.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0046] (First Embodiment)

[0047]FIG. 1 is a layout diagram showing the schematic structure of a polishing apparatus of substrate according to the first embodiment of the present invention, FIG. 2 a view showing the structure of a polishing mechanism section in the polishing apparatus of substrate according to the first embodiment of the present invention, and FIG. 3 a view showing the structure of the main part of the same polishing mechanism section.

[0048] The polishing apparatus of substrate according to the present invention is provided, as shown in FIG. 1, with a wafer polishing section 51 for polishing a substrate, such as a semiconductor substrate, a glass substrate, or the like (which will be also referred to simply as a wafer), as a work, a cleaning section 52 for cleaning the wafer after polished in the wafer polishing section 51, a prealignment section 53 for effecting prealignment of the wafer after cleaned in the cleaning section 52, and a film symmetry measuring section 54 for measuring symmetry of a film on alignment marks of the wafer after the prealignment. Numeral 55 designates an XYθ stage, which holds the wafer cleaned in the cleaning section 52 and which transfers the wafer to the prealignment section 53 and the film symmetry measuring section 54. The prealignment section 53 performs positioning in the rotational direction on the basis of a notch reference or an orientation flat reference and positioning in the XY directions on the basis of a wafer contour reference, and the film symmetry measuring section 54 measures symmetry of the film on the alignment marks. Numeral 56 denotes a wafer load-unload section for loading a wafer stored in a wafer carrier into the polishing apparatus and unloading the wafer from the apparatus, and numeral 57 a carry robot for carrying the wafer.

[0049] Next, the polishing mechanism section of wafer in the polishing apparatus will be described in detail with reference to FIG. 2 and FIG. 3. The wafer polishing mechanism section is provided with a wafer table 2 for holding a wafer W as a work through a wafer chuck 1 with a polished surface of the wafer facing up, and a polishing head 5 which is disposed above the wafer table 2 so as to face the wafer W held on the wafer table 2 and which has the diameter larger than the diameter of the wafer W but smaller than double the diameter of the wafer W. The polishing mechanism section is further provided with a first driver 7 for rotationally driving the polishing head 5 holding the polishing pad P in the direction of arrow A about the axis thereof and a head vertical motion driver 8 for vertically moving the polishing head 5 to press the polishing pad P against the wafer W under pressure. On the wafer table 2 holding the wafer W, as shown in FIG. 3, there are provided a second driver 10 for rotationally driving the wafer chuck 1 holding the wafer W, in the direction of arrow B about the axis thereof, a third driver 11 consisting of a guide section 11 a and a power section 11 b for rocking the wafer chuck 1 holding the wafer W, in horizontal directions (arrow C), and an equalizing mechanism 12 for pressing the entire surface of the wafer W against the polishing pad P under a fixed pressure on the occasion of polishing the wafer W by the polishing pad P. The polishing mechanism section is further provided with an abrasive supply mechanism 16 having an abrasive supply tube 15 in communication with a small hole 14 provided in the central area of the polishing head 5 and the polishing pad P so as to supply the abrasive (slurry) to the region where the polished surface of the wafer W and the polishing pad P face each other. If the polishing pad P is made of a material that the abrasive S can penetrate, e.g., fabric or polyurethane having large communicating holes, the small hole 14 does not have to be provided intentionally in the polishing pad P.

[0050] The polishing mechanism section of the present embodiment is further provided with a control unit 17 for controlling driving of the foregoing first driver 7, head vertical motion driver 8, second driver 10, third driver 11, etc. independently of each other or in correlation with each other. The control unit 17 is composed of a microcomputer and the like.

[0051] In the polishing mechanism section of the present embodiment, the axis of autorotation of the polishing pad P is set off the axis of autorotation of the wafer W, i.e., the axes of the polishing pad P and the wafer W are located at different axis positions, and the pad and the wafer are arranged so that the entire surface of the wafer W never lies off the polishing pad P. The polishing mechanism section is also constructed to keep the wafer W from lying off the polishing pad P during rocking. In this way the polishing mechanism section performs the polishing in a state in which the polishing pad P is always in contact with the whole of the polished surface of the wafer W. For that purpose, in the present embodiment, the positions of the respective autorotation axes are set so that the sum of the radius of the wafer W and the distance between the autorotation axis of the polishing pad P and the autorotation axis of the wafer W is not more than the radius of the polishing pad P. For rocking the wafer W, the rocking range is also determined so that the sum of the radius of the wafer W and the maximum distance between the autorotation axes during the rocking is not more than the radius of the polishing pad P.

[0052] In the present embodiment, as shown in FIG. 3, the wafer W is rocked by the third driver 11, but it is also possible to construct the polishing mechanism section so as to rock the polishing head P side by a rocking means provided on the polishing head P side, or to construct the polishing mechanism section so as to rock both the wafer W and the polishing head P. In either case, however, it is necessary to keep the wafer W from lying off the polishing pad P.

[0053] The abrasive used in the present embodiment is a polishing solution in which fine particles, a material of which is selected from silicon oxide, cerium oxide, aluminum oxide, oxide zeolite, chromium oxide, iron oxide, silicon carbide, boron carbide, carbon, ammonium salt, and so on and diameters of which are relatively uniform in the range of several micron order to submicron order, are dispersed in a solution selected from aqueous sodium hydroxide solution, aqueous potassium hydroxide solution, aqueous ammonia solution, isocyanurate solution, Br-CH₃OH, hydrochloric acid solution, and so on. The combination of these fine particles and solution can be selected according to an object to be polished. For example, the abrasive suitably applicable to polishing of Si surface is one in which fine particles of silicon oxide, cerium oxide, ammonium salt, manganese dioxide, or the like are dispersed in the above-mentioned solution. The abrasive suitably applicable to polishing of surface of SiO₂ is one in which fine particles of silicon oxide are dispersed in aqueous potassium hydroxide solution. The abrasive suitably applicable to polishing of an Al-surface wafer is one in which fine particles of silicon oxide are dispersed in aqueous ammonia solution containing hydrogen peroxide. In the present invention, films to be polished can be insulators such as silicon oxide, silicon nitride, nitrided silicon oxide, and the like, and these may be doped with boron or phosphorus if necessary.

[0054] Next, the polishing method by the polishing apparatus constructed as described above will be described below along the flowchart shown in FIG. 4. In the present embodiment the polishing is implemented by a control method of driving the wafer W as a work and the polishing pad P in the same direction and at different rotation speeds, and the respective rotation speeds of the wafer W and the polishing pad P are selected in the range of not more than 1000 rpm and more preferably in the range of 50 to 300 rpm in the same direction. The pressure for pressing the polishing pad P against the wafer W by the head vertical motion driver 8 is set in the range of 0 to 100 kPa.

[0055] The wafer is a substrate in which an insulating film is formed on a device forming surface thereof so as to cover alignment marks.

[0056] First, in step S1, a wafer W stored in the wafer carrier placed in the wafer load-unload section 56 is carried to the wafer polishing section 51 by the carry robot 57 according to a wafer loading sequence and the surface of the insulating film formed on the substrate is polished there (step S3). The mode of polishing is as described above, and for the polishing, the polishing pressure, the polishing period, the rotation speed of the wafer, the rotation speed of the polishing pad, etc. are preliminarily set to arbitrary values and are inputted through unrepresented input means. The polishing is carried out according to these input values. Desirably, the difference n between the rotation speeds of the wafer and the polishing pad is approximately 1 to 10 rpm. The wafer W after completion of the polishing is conveyed from the wafer polishing section 51 to the cleaning section 52 by the carry robot 57 and the wafer W is cleaned and dried in the cleaning section 52 (step S4). After that, the wafer is transferred onto the XYθ stage 55 by the carry robot 57. The wafer W on the XYθ stage 55 is transferred to the prealignment section 53 and in the prealignment section 53 the wafer W is subjected to the alignment in the rotational direction on the basis of the notch reference or the orientation flat reference and the positioning in the XY directions on the basis of the wafer contour reference as prealignment (step S5). Then the wafer W is transferred to the film symmetry measuring section 54 by the XYθ stage 55. According to the positions and number of alignment marks preliminarily inputted in step S6, the XYθ stage 55 is driven in the XY directions to detect the alignment marks (step S7), and thereafter, in step S8, symmetry is measured for the polished insulating film on the alignment marks.

[0057] An alignment mark symmetry detecting system 60 shown in FIG. 5 is used for the measurement of symmetry of the insulating film on the alignment marks m. The alignment mark symmetry detecting system 60 is configured to direct light emitted from a light source 62, through an optical system 63 including a beam splitter, to an alignment mark m on the wafer W and focus reflected light therefrom through the optical system 63 on CCD 61 as an image pickup device. A signal processing system processes an electric signal of the alignment mark image focused on the CCD 61, thereby measuring symmetry of the film on the alignment mark m. The measurement of symmetry of film will be detailed later. The structure of the optical system of this symmetry detecting system 60 is basically similar to that of the bright field optical system of FIG. 15 described previously. The number of alignment marks m to be measured can be preliminarily set to an arbitrary number from the unrepresented computer section, and preferably set to approximately 2 to 18. On that occasion, the correction accuracy can be enhanced by selecting a plurality of alignment marks on an identical circle.

[0058] Based on the measured symmetry of each alignment mark thus selected, the result is fed back to the difference between rotation speeds of wafer W and polishing pad P for polishing of a next wafer W (step S9), to appropriately correct the difference between the rotation speeds of the polishing pad P and the next polished wafer W (step S2). After completion of the measurement of symmetry of alignment marks, the wafer W is unloaded according to a wafer unloading sequence to be stored in a wafer carrier in the wafer load-unload section 56 by the carry robot 57 (step S10).

[0059] Next, the measurement of symmetry of the film on the alignment marks will be further described with reference to FIGS. 6A and 6B. When template matching is conducted so as to set the measurement range for the alignment mark m imaged on the CCD 61 to the positional relation of FIG. 6A, a measurement signal shown in FIG. 6B is gained. The measurement signal obtained from the alignment mark m in the central part of the measurement range varies in a shape of two peaks in the measurement direction. The measurement signal is sliced at a certain slice level to obtain two intersections and determine an intermediate point between them for each peak, a middle point between two intermediate points is defined as a center position of the alignment mark, and a difference between distances from that center position to tops of the respective peaks is defined as symmetry of the alignment mark.

[0060] Another method of measurement of symmetry is a method of folding the measurement signal varying in the two-peak shape at the center and taking a difference. The following measurement is carried out for x of the measurement signal obtained as shown in FIG. 6B. $\begin{matrix} {{M(x)} = {{\sum\limits_{j = {C - {W/2}}}^{C + {W/2}}{F\left( {x + j} \right)}} - {F\left( {x - j} \right)}}} & (1) \end{matrix}$

[0061] A minimum is determined out of absolute values of M(x) obtained by Eq (1) above and the minimum is divided by W. The resultant of the division is defined as symmetry level.

symmetry level =M(x)/W. . .(2)

[0062] In the above equations, W and C represent coefficients determined by the type of the alignment mark, the magnification of the detection optical system, the number of pixels of CCD, and so on.

[0063] Further, still another method of measurement of symmetry is a method of comparing the actually measured signal with a waveform with good symmetry preliminarily stored in a computer and calculating the symmetry level, based on correlation between them.

[0064] Described next is a method of correcting the difference n between the rotation speeds of the wafer and polishing pad, based on the symmetry level measured as described above.

[0065] When the polishing is carried out with the shift of several % between the rotation speeds (rpms) of the wafer and polishing pad, the film on the alignment marks is not polished isotropically and thus the alignment mark detecting system measures positional deviation. However, when the polishing is carried out with the shift of several % between the rotation speeds of the wafer and polishing pad, it becomes feasible on the other hand to correct the positional deviation of the alignment marks due to the difference between the rotation speeds of the wafer and polishing pad, the temporal deformation of the polishing pad, and so on. In the present embodiment, thus, actual polishing is preliminarily carried out with change in the rotation speeds of the respective wafer and polishing pad to several values, for films like insulating films on the alignment marks on the wafer in the device process as an object to be polished, and, based on results of measurement of the films on the alignment marks at that time, the relation between symmetry level and difference between rotation speeds of the wafer and polishing pad is statistically processed and preliminarily stored in a computer. Then the symmetry level obtained by the measurement of symmetry is used for the operation in the computer to compute the difference n between rotation speeds of the wafer and polishing pad to be changed according to the measured symmetry level.

[0066] In the semiconductor fabrication process, it is common practice to first run a test wafer for checking stability of the system at the beginning of a fabrication lot. In the CMP apparatus, a test wafer is also polished before polishing of the first wafer, in order to check the stability of the expendable materials such as the polishing pad, the abrasive, etc., thereby checking the polishing rate, flatness, and so on. Then, the symmetry of the film on the alignment marks is measured on the occasion of the polishing of this test wafer and the rotation speed difference n can be calculated according to this symmetry. Using the rotation speed difference n calculated for this test wafer, the polishing is carried out by setting the rotation speed of the wafer to N and the rotation speed of the polishing pad to (N −n). This permits the film on the alignment marks to be polished in symmetry, thereby improving the alignment accuracy of wafer.

[0067] As described above, the polishing apparatus incorporates the film symmetry measuring unit for measuring the symmetry of the film on the alignment marks and is constructed to perform the polishing while feeding back the data of the wafer polished one before, whereby, even with change in such a factor as the temporal change or the like of the polishing pad during processing of one lot, the film on the alignment marks can be polished in symmetry in accordance with the change. This improves the alignment accuracy, prevents the transfer of the grid-like groove pattern scribed in the surface of the polishing pad, and enhances the microflatness.

[0068] (Second Embodiment)

[0069] The polishing method and apparatus of substrate according to the second embodiment of the present invention will be described below with reference to FIG. 7 and FIG. 8.

[0070] There are two conceivable reasons for the problem of degradation of the alignment accuracy in the alignment detecting system of the overlay inspection system, the stepper, etc. due to the asymmetric polishing of the film on the alignment marks in the polishing of the wafer. The first reason is that the film on the alignment marks is not polished isotropically, so as to become asymmetric, because the polishing is conducted with the shift of several % between the rotation speeds (rpms) of the wafer and polishing pad. The second reason is that the film on the alignment marks is not polished isotropically, so as to become asymmetric, because of the temporal deformation of the polishing pad.

[0071] In the present embodiment, thus, the rotation speeds of the wafer and polishing pad are controlled in order to solve the former and the difference between the rotation speeds of the wafer and polishing pad is corrected in order to solve the latter. The polishing apparatus used in the present embodiment is approximately similar to the polishing apparatus described in the foregoing first embodiment (FIG. 1 to FIG. 3) and the details of like members are omitted herein.

[0072] First described is a method of controlling the rotation speeds of the wafer and polishing pad in the present embodiment.

[0073] When the wafer W and the polishing pad P are driven in the same direction and at the same rotation speed as an ordinary polishing method in polishing of wafers and the like, relative peripheral velocity can be kept uniform at an arbitrary position on the polished surface of wafer W and this enables implementation of polishing while ensuring global uniformity. However, when the wafer W and polishing pad P are driven in the same direction and at the same rotation speed, satisfactory microflatness cannot be attained, because the grid-like groove pattern for abrasive in the surface of the polishing pad P is transferred to the wafer W. For this reason, normally, the polishing is done with the shift of several % between the rotation speeds of the wafer W and polishing pad P. The range of the shift between the rotation speeds in this way is approximately 0.1 to 10% relative to the rotation speed of either one member and more preferably about 1 rpm. For example, let us suppose that while the wafer W is rotated at 60 rpm, the polishing pad P is rotated at 59 rpm and the film is polished for one minute. In this case, the wafer W undergoes 60 rotations in total and the polishing pad P 59 rotations in total. Thus the difference between the total numbers of rotations of the wafer W and the polishing pad P is calculated as 60−59=1 rotation and this is reduced to an angular rotation difference of 360°. When the grooves for abrasive scribed in the polishing pad P are of the grid shape as described previously, the transfer of the groove pattern can be prevented if the rotation difference is not less than 90°. If the grooves for abrasive scribed in the polishing pad P are spiral, the transfer of the groove pattern can be averaged by the rotation difference of 360°.

[0074] Incidentally, when the polishing is simply done with the shift of several % between the rotation speeds of the wafer W and polishing pad P, asymmetric polishing results from the rotation speed difference. The mechanism of this polishing will be described next. The relation between relative positions of the wafer W and polishing pad P during rotation of the wafer W and polishing pad P in the same direction and at the same rotation speed is equal to that during revolving motion without autorotation, the radius of which is the distance between the autorotation axis of the wafer W and the autorotation axis of the polishing pad P. In this case, the film layer on the alignment marks m on the wafer W is polished isotropically. However, when the polishing is done with the shift of several % between the rotation speeds of the wafer W and the polishing pad P, autorotation motion occurs according to a difference between their rotation numbers and the film on the alignment marks m on the wafer W is polished on an asymmetric basis. Accordingly, there occurs positional deviation of the alignment marks as shown in FIG. 17.

[0075] Thus the polishing apparatus of the present embodiment further comprises a control means for controlling the rotation of the wafer W and polishing pad P so as to change the rotation speeds (rpms) of the wafer W and the polishing pad P and equate the total numbers of rotations of the wafer W and the polishing pad P from the start of the polishing to the end of the polishing, this control means comprises an input means for setting the rotation speed of the wafer W, the rotation speed of the polishing pad P, the polishing period, and the polishing pressure and an input means for setting the rotation speed switching time, and the control means further comprises an operation part for executing such calculation as to equate the numbers of rotations of the wafer W and polishing pad P and a control part for driving the wafer W and the polishing pad P at their respective, predetermined rotation speeds.

[0076] The control of the present embodiment will be described below in detail with reference to FIG. 7 and FIG. 8. As shown in the timing chart of FIG. 7, let T be the total polishing period, t be the rotation speed switching time, N₁ (rpm) be the rotation speed of the wafer W before the rotation speed switching time t, N₂ (rpm) be the rotation speed of the wafer W after the rotation speed switching, (N₁−n) (rpm) be the rotation speed of the polishing pad P before the rotation speed switching time t, and N_(x) (rpm) be the rotation speed of the polishing pad P after the rotation speed switching. Then the rotation speed N_(x) (rpm) of the polishing pad P after the rotation speed switching can be calculated by the equation below. Namely, from the fact that in the total polishing period from the start of polishing to the end of polishing the total number of rotations of the wafer W, {N₁·t+N₂·(T−t)}, is equated to the total number of rotations of the polishing pad P, {(N₁−n)·t+N_(x)·(T−t)}, the following equation holds.

N₁·t+N₂·(T−t)=(N₁−n)·t+N_(x)·(T−t)

[0077] Therefore, we obtain the following equation.

N_(x)=[N₁·t+N₂·(T−t)−(N₁−n)·t]/(T−t) . . . (3)

[0078] Namely, by giving input of respective values of the total polishing period T, the rotation speed switching time t, the rotation speed N₁ (rpm) of the wafer W before the rotation speed switching time t, the rotation speed N₂ (rpm) of the wafer W after the rotation speed switching, and the rotation speed (N₁−n) (rpm) of the polishing pad P before the rotation speed switching time t by use of the input means, the operation part can calculate the rotation speed N_(x) (rpm) of the polishing pad P after the rotation speed switching, based on Eq (3).

[0079] For example, suppose the total polishing period T is two minutes, the rotation speed switching time t one minute, the rotation speeds N₁, N₂ of the wafer W before and after the rotation speed switching time t are both 60 rpm, and the rotation speed (N₁ −n) of the polishing pad P before the rotation speed switching time t is (60−1=) 59 rpm. Then the rotation speed N_(x) (rpm) of the polishing pad P after the rotation speed switching is calculated as follows by Eq (3) described above.

N_(x)={60×1+60 ×(2−1)−59×1}/(2−1)=61

[0080] At this time, the numbers of rotations of the wafer W and the polishing pad P are given as follows. the number of rotations of the wafer ={60×1+60×(2−1)}=120 the number of rotations of the polishing pad ={59 ×1+61×(2−1)}120

[0081] Namely, though polishing is carried out with the shift of several % between the rotation speeds of the wafer W and the polishing pad P, the difference is 0 between the numbers of rotations of the wafer W and the polishing pad P, so that isotropic polishing is implemented.

[0082] When the rotation speed switching time t is half of the polishing period T and rotating directions are switched (e.g., from CW to CCW) between before and after the switching, velocity vectors at each part on the wafer W cancel each other before and after the switching time, thereby enabling implementation of further more isotropic polishing.

[0083] It is needless to mention that the effect of the present embodiment can also be achieved similarly by exchanging the speeds of the wafer W and the polishing pad P in foregoing Eq (3) and calculating the speed of the wafer W after the switching time, as N_(x).

[0084] The apparatus may also be configured to store the rotation speed switching time t, the speed N₂ after switching, and the difference n between rotation speeds of the wafer W and polishing pad P as coefficients in the operation part.

[0085] A method of correction for the rotation speed difference n between the rotational speeds of the wafer W and the polishing pad P will be described below.

[0086] Against the phenomenon in which the film on the alignment marks is not polished isotropically because of the difference between the rotation speeds of the wafer W and the polishing pad P, the desired effect can be achieved, as described above, by controlling the rotation of the wafer W and the polishing pad P so as to equate the total numbers of rotations thereof. However, even if the rotation of the wafer W and polishing pad P is controlled so as to equate the total numbers of rotations thereof, there are cases where the film on the alignment marks is not polished isotropically because of the temporal deformation of the polishing pad P. Therefore, the difference n between the rotation speeds of the wafer W and the polishing pad P is corrected according to the symmetry of the film on the alignment marks after polishing, whereby the film on the alignment marks can be polished isotropically.

[0087] This will be described using the flowchart shown in FIG. 8. The polishing apparatus in the present embodiment is approximately similar to the polishing apparatus shown in FIG. 1 to FIG. 3, as described previously, and thus reference is also made to FIG. 1 to FIG. 3 in the description.

[0088] First, a wafer W is loaded into the polishing apparatus (step S21) and then is conveyed to the wafer polishing section 51. The input means also provides input of the preset total polishing period T, rotation speed switching time t, rotation speed N₁ (rpm) of the wafer W before the rotation speed switching time, rotation speed N₂ (rpm) of the wafer W after the rotation speed switching, and rotation speed (N₁−n) (rpm) of the polishing pad P before the rotation speed switching time (step S22), and then the rotation speed N_(x) of the polishing pad P after the rotation speed switching is calculated according to aforementioned Eq (3) on the basis of the input values (step S23). N_(x) is set to a value satisfying |N_(x)−N₂|≧1 (steps S23 to S25). Based on the values inputted and calculated as described above, the wafer W is polished in step S26. After completion of the polishing, the wafer W is cleaned and dried in the cleaning section 52 (step S27) and thereafter, the wafer W is transferred onto the XYθ stage 55 and moved to the prealignment section 53. In the prealignment section 53 the wafer W is positioned by the alignment in the rotational direction on the basis of the notch reference or the orientation flat reference and the positioning in the XY directions on the basis of the wafer contour reference as prealignment (step S28).

[0089] Then the wafer W is transferred to the film symmetry measuring section 54 by the XYθ stage 55 and, according to the positions and the number of alignment marks preliminarily inputted in step S29, the XYθ stage 55 is driven in the XY directions to detect the alignment marks in step S30. After that, symmetry of the film on the alignment marks is measured in step S31. A corrected rotating speed N₃ for a next polished wafer W and the polishing pad P is calculated according to the symmetry thus measured (step S32). This corrected rotating speed N₃ is used for determining the rotation speed N_(x) of the polishing pad P after the rotation speed switching in the polishing of the next wafer W, as described hereinafter. After completion of the measurement of symmetry of the alignment marks, the wafer W is unloaded to be stored in the wafer carrier of the wafer load-unload section 56 (step S33).

[0090] The corrected rotating speed N₃ described above can be obtained in similar fashion to the calculation of the rotation speed difference n in the first embodiment. Namely, for films on the alignment marks m on the wafer W as an object to be polished, actual polishing is preliminarily carried out with change in each rotation speed of the wafer W and the polishing pad P to several values and, based on results of the measurement of the films on the alignment marks m at that time, the relation between symmetry level and rotation speed difference between the wafer W and the polishing pad P is statistically processed and stored in a computer. Then the symmetry level obtained by the measurement of symmetry is used for the operation in the computer to calculate the corrected rotating speed N₃ according to the measured symmetry level.

[0091] For determining the rotation speed N_(x) of the polishing pad P after the rotation speed switching in the polishing of the next wafer W by use of the corrected rotating speed N₃, as shown in the timing chart of FIG. 7, let T be the total polishing period, t be the rotation speed switching time, N₁ (rpm) be the rotation speed of the wafer W before the rotation speed switching time, N₂ (rpm) be the rotation speed of the wafer W after the rotation speed switching, and (N₁−n) (rpm) be the rotation speed of the polishing pad P before the rotation speed switching time. Then the rotation speed N_(x) (rpm) of the polishing pad P after the rotation speed switching can be calculated by Eq (4) below, using the corrected rotating speed N₃.

N_(x)=[N₁·t+N₂·(T−t)−(N₁−n)·t+N₃]/(T−t). . . (4)

[0092] In the present embodiment, as described above, the rotation speeds of the wafer and the polishing pad are controlled to implement the isotropic polishing of the film on the alignment marks and the data of the wafer polished one before is fed back whereby, even with change in such a factor as the temporal change of the polishing pad or the like, the film on the alignment marks can be polished in symmetry according to the change. This improves the alignment accuracy and prevents the transfer of the grid-like groove pattern scribed in the surface of the polishing pad, thereby improving the microflatness.

[0093] Although in the first and second embodiments the film symmetry measuring unit for measuring the symmetry of the alignment marks for alignment is disposed in the polishing apparatus, it can also be contemplated that the film symmetry measuring unit is located outside the polishing apparatus, the wafer after completion of polishing is subjected to the measurement of symmetry outside the polishing apparatus, and the data of symmetry level obtained is inputted through the input part provided in the polishing apparatus.

[0094] (Third Embodiment)

[0095] The polishing method and apparatus of semiconductor substrate according to the third embodiment of the present invention will be described below with reference to FIG. 9 and FIG. 10.

[0096] In the third embodiment of the present invention, the symmetry of the film on the alignment marks is measured in the middle of polishing of a wafer and respective rotation speeds of the wafer and polishing pad for the rest polishing are controlled according to the symmetry of the film on the alignment marks as the result of the measurement.

[0097] As shown in the timing chart of FIG. 9, the symmetry of film on the alignment marks is measured after the first half of polishing or primary polishing (which will be referred to hereinafter simply as primary polishing), and based on the result, the rest half of polishing or secondary polishing (which will be referred to hereinafter simply as secondary polishing) is carried out. In this case, let T_(f)+T_(s) be the total polishing period, T_(f) be a primary polishing period before the measurement of symmetry, t_(f) be a rotation speed switching time in the primary polishing before the measurement of symmetry, N_(1f) (rpm) be a rotation speed of the wafer W before the rotation speed switching time in the primary polishing, N_(2f) (rpm) be a rotation speed of the wafer W after the rotation speed switching in the primary polishing, (N_(1f)−n_(f)) (rpm) be a rotation speed of the polishing pad P before the rotation speed switching time in the primary polishing, T_(s) be a secondary polishing period after the measurement of symmetry, t_(S) be a rotation speed switching time in the secondary polishing after the measurement of symmetry, N_(1s) (rpm) be a rotation speed of the wafer W before the rotation speed switching time in the secondary polishing after the measurement of symmetry, N_(2s) (rpm) be a rotation speed of the wafer W after the rotation speed switching in the secondary polishing, and (N_(1s)−n_(s)) (rpm) be a rotation speed of the polishing pad P before the rotation speed switching time in the secondary polishing. When these are set as input values, a rotation speed N_(xf) (rpm) of the polishing pad P after the rotation speed switching time in the primary polishing before the measurement of symmetry and a rotation speed N_(xs) (rpm) of the polishing pad P after the rotation speed switching time in the secondary polishing after the measurement of symmetry can be calculated according to respective equations below. In the equations, N₃ is a corrected rotating speed similar to that in the aforementioned second embodiment, and is a corrected rotating speed calculated according to the symmetry measured by the measurement of symmetry of the film on the alignment marks.

N_(xf)=[N_(1f)·t_(f)+N_(2f)·(T_(f)−t_(f))−(N_(1f)−n_(f))·t_(f)]/(T_(f)−t_(f)). . . (5)

N_(xs)=[N_(1s)·t_(s)+N_(2s)·(T_(s)−t_(s))−(N_(1s)−n_(s))·t_(s)+N₃]/(T_(s)−t_(s)). . . (6)

[0098] Preferably, N_(1f)=N_(2f) and t_(f) is preferably half of T_(f). Further, when T_(f)=T_(s), t_(f)=t_(s),N_(1f)=N_(1s), N_(2f)=N_(2s), and (N_(1f)−n_(f)) =(N_(1s)−n_(s)), velocity vectors on the wafer during polishing become more isotropic, so as to improve the symmetry further.

[0099] In the present embodiment, the symmetry of the film on the alignment marks is measured in the middle of the polishing and the result is fed back to the polishing of the wafer itself, whereby the isotropic polishing is implemented for each wafer without use of a test wafer.

[0100] In general CMP involves the primary polishing being rough polishing and the secondary polishing being finish polishing. In this case, when the symmetry of the film on the alignment marks is measured as described above between the rough polishing and the finish polishing and the measurement result is fed back to the finish polishing, it becomes feasible to isotropically polish the film on the alignment marks. Namely, as shown in FIG. 10, the measurement of symmetry of the film on the alignment marks, as well as detection of an end point, is carried out between the primary polishing and the secondary polishing. In FIG. 10, a wafer is loaded in the polishing apparatus (step S41), the wafer is polished by the primary polishing (step S42), then the wafer is prealigned (step S43), and thereafter, the end is detected (step S44) and the symmetry of the film on the alignment marks is also measured (steps S45 to S46). The measurement result is fed back to the secondary polishing carried out in the subsequent step (step S47). After completion of the secondary polishing, the wafer is cleaned and dried (step S48) and is unloaded (step S49).

[0101] As described above, the symmetry of film on the alignment marks is measured in the middle of the polishing and the result of the measurement is fed back to the polishing of the wafer itself whereby it becomes feasible to implement the isotropic polishing for each wafer, polish the film on the alignment marks in symmetry, and enhance the alignment accuracy.

[0102] As described above, the present invention enables the isotropic polishing of the film on the alignment marks for alignment of the overlay inspection system, the stepper, etc. on the polished surface of the semiconductor substrate such as the wafer or the like. This permits the film on the alignment marks to be polished in symmetry, improves the alignment accuracy, and prevents the transfer of the grid-like groove pattern scribed in the surface of the polishing pad, thereby improving the microflatness and increasing the yields throughout the entire production steps of semiconductor devices. 

What is claimed is:
 1. A polishing method comprising the steps of keeping a polishing pad under a predetermined pressure and in a state of contact with a device forming surface of a substrate and performing polishing while rotating each of said substrate and said polishing pad, wherein symmetry of a film on an alignment mark for alignment of said substrate is measured after an end of the polishing of said substrate or in the middle of the polishing and wherein a rotation speed of at least either one of said substrate and said polishing pad is controlled according to the symmetry of the film on the alignment mark, thus measured.
 2. The polishing method according to claim 1, wherein control is implemented so as to change rotation speeds of said substrate and said polishing pad and equate total numbers of rotations of said substrate and said polishing pad from a start of the polishing to the end of the polishing.
 3. The polishing method according to claim 1, which comprises setting a rotation speed of said substrate, a rotation speed of said polishing pad, a polishing period, and a polishing pressure and further setting a rotation speed switching time, performing such calculation as to equate total numbers of rotations of said substrate and said polishing pad from a start of the polishing to the end of the polishing, based on these set rotation speed of said substrate, rotation speed of said polishing pad, polishing period, and rotation speed switching time, and driving said substrate and said polishing pad at respective rotation speeds based on the result of the calculation.
 4. The polishing method according to claim 1, which comprises storing a rotation speed switching time and either one or both of rotation speeds of said substrate and said polishing pad and performing such control as to equate total numbers of rotations of said substrate and said polishing pad from a start of the polishing to the end of the polishing.
 5. A polishing apparatus comprising driving means for rotating a substrate and driving means for rotating a polishing pad and constructed to keep the polishing pad under a predetermined pressure and in a state of contact with a device forming surface of said substrate and perform polishing while rotating each of said substrate and said polishing pad, said polishing apparatus comprising measuring means for measuring symmetry of a film on an alignment mark for alignment of said substrate or an input part for inputting symmetry of a film on an alignment mark, and control means for controlling a rotation speed of at least either one of said substrate and said polishing pad according to the symmetry of the film on the alignment mark, thus measured or inputted.
 6. The polishing apparatus according to claim 5, wherein said control means performs such control as to change rotation speeds of said substrate and said polishing pad and equate total numbers of rotations of said substrate and said polishing pad from a start of the polishing to an end of the polishing.
 7. The polishing apparatus according to claim 5, wherein said control means comprises first input means for setting a rotation speed of said substrate, a rotation speed of said polishing pad, a polishing period, and a polishing pressure and second input means for setting a rotation speed switching time, and said control means further comprises an operation part for performing such calculation as to equate total numbers of rotations of said substrate and polishing pad from values inputted by said first and second input means, and a control part for driving each of said substrate and said polishing pad at respective rotation speeds based on the rotation speeds inputted by said first input means and the result of the calculation by said operation part.
 8. The polishing apparatus according to claim 5, which comprises a memory section for storing a rotation speed switching time and either one or both of rotation speeds of said substrate and said polishing pad, wherein control is implemented so as to equate total numbers of rotations of said substrate and said polishing pad from a start of the polishing to an end of the polishing.
 9. A device fabrication method comprising: a step of forming a film covering an alignment mark for alignment of a substrate on a device forming surface of said substrate; and a step of keeping a polishing pad under a predetermined pressure and in a state of contact with said film and performing polishing while rotating each of said substrate and polishing pad; said device fabrication method comprising a step of measuring symmetry of the film on said alignment mark after an end of the polishing of said substrate or in the middle of the polishing and controlling a rotation speed of the substrate and/or the polishing pad according to the symmetry of the film on said alignment mark, thus measured.
 10. The device fabrication method according to claim 9, wherein control is implemented so as to change rotation speeds of said substrate and said polishing pad and equate total numbers of rotations of said substrate and said polishing pad from a start of the polishing to the end of the polishing.
 11. The device fabrication method according to claim 9, which comprises setting a rotation speed of said substrate, a rotation speed of said polishing pad, a polishing period, and a polishing pressure and further setting a rotation speed switching time, performing such calculation as to equate total numbers of rotations of said substrate and said polishing pad from a start of the polishing to the end of the polishing, based on these set rotation speed of said substrate, rotation speed of said polishing pad, polishing period, and rotation speed switching time, and driving said substrate and said polishing pad at respective rotation speeds based on the result of the calculation.
 12. The device fabrication method according to claim 9, which comprises storing a rotation speed switching time and either one or both of rotation speeds of said substrate and said polishing pad and performing such control as to equate total numbers of rotations of said substrate and said polishing pad from a start of the polishing to the end of the polishing. 